Radio frequency front-end

ABSTRACT

A radio frequency front-end is disclosed having a first power amplifier (PA) having a first PA input and a first PA output, a second PA having a second PA input and a second PA output, and a low-noise amplifier (LNA) having an LNA output connected to a receive output terminal and an LNA input. An input 90° hybrid coupler has a first port input connected to a transmit terminal, a second port input connected to a fixed voltage node through an isolation impedance, a third port output connected to the first amplifier input and a fourth port output connected to the second amplifier input. An output 90° hybrid coupler has a first port output connected to a common terminal, a second port output connected to the LNA input, a third port input connected to the second PA output, and a fourth port input connected to the first PA output.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 63/114,169, filed Nov. 16, 2020, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure pertains to radio frequency front-ends havingpower amplifiers, low-noise amplifiers, and switches for transmit andreceive operations.

BACKGROUND

Radio frequency front-ends typically include at least one poweramplifier, a low-noise amplifier, and a single-pole double-throw switch.The single-pole double-throw switch is used to switch a common portbetween transmit and receive, where in transmit the power amplifieroutput is routed to the common port and in receive the low-noiseamplifier input is routed to the common port. As frequency, power level,and operating bandwidths increase, the design of the single-poledouble-throw switch becomes quite challenging. The current handling ofon-state switch field-effect transistor devices becomes problematic athigh power and high frequency because switch field-effect transistorperiphery cannot be arbitrarily increased while maintaining acceptableperformance. What is needed is a radio frequency front-end that reducesthe required current handling of the on-state switch devices and inaddition improves receive-transmit isolation.

SUMMARY

A radio frequency front-end is disclosed having a first power amplifier(PA) having a first PA input and a first PA output, a second PA having asecond PA input and a second PA output, and a low-noise amplifier (LNA)having an LNA output connected to a receive output terminal and an LNAinput. Also included is an input 90° hybrid coupler having a first portinput connected to a transmit terminal, a second port input connected toa fixed voltage node through an isolation impedance, a third port outputconnected to the first amplifier input, and a fourth port outputconnected to the second amplifier input. Further included is an output90° hybrid coupler having a first port output connected to a commonterminal configured to output transmit signals and input receivesignals, a second port output connected to the LNA input, a third portinput connected to the second PA output, and a fourth port inputconnected to the first PA output.

In another aspect, any of the foregoing aspects individually ortogether, and/or various separate aspects and features as describedherein, may be combined for additional advantage. Any of the variousfeatures and elements as disclosed herein may be combined with one ormore other disclosed features and elements unless indicated to thecontrary herein.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure and,together with the description, serve to explain the principles of thedisclosure.

FIG. 1 is a diagram showing a related-art balanced power amplifierarchitecture.

FIG. 2 is a diagram of a radio frequency front-end that is structured inaccordance with the present disclosure.

FIG. 3 is a diagram of an embodiment of the radio frequency front-endwith shunt switches to increase receive-transmit isolation and reducereceive loss, according to the present disclosure.

FIG. 4 is a plot showing additional loss versus the magnitude of thereflection coefficient.

FIG. 5 is a plot more focused on the higher reflection coefficientterminations.

FIG. 6 is a plot showing shunt resistance versus the loss.

FIG. 7 is a plot showing shunt resistance versus the loss, but with afocus on the useful region.

FIG. 8 is a plot showing the magnitude of the reflection coefficient fora shunt switch field-effect transistor fabricated in gallium nitride(GaN) biased to the on-state at 30 GHz.

FIG. 9 is a plot showing the magnitude of the reflection coefficient fora shunt switch field-effect transistor fabricated in GaN biased to theon-state at 20 GHz.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Embodiments are described herein with reference to schematicillustrations of embodiments of the disclosure. As such, the actualdimensions of the layers and elements can be different, and variationsfrom the shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are expected. For example, aregion illustrated or described as square or rectangular can haverounded or curved features, and regions shown as straight lines may havesome irregularity. Thus, the regions illustrated in the figures areschematic and their shapes are not intended to illustrate the preciseshape of a region of a device and are not intended to limit the scope ofthe disclosure. Additionally, sizes of structures or regions may beexaggerated relative to other structures or regions for illustrativepurposes and, thus, are provided to illustrate the general structures ofthe present subject matter and may or may not be drawn to scale. Commonelements between figures may be shown herein with common element numbersand may not be subsequently re-described.

FIG. 1 is a diagram showing a related-art balanced power amplifier 10.The power amplifier 10 is implemented as a balanced amplifier where aninput signal is split in quadrature between a first power amplifier (PA)12 labeled PA1 and a second PA 14 labeled PA2 by way of an input 90°hybrid coupler 16 configured as a signal splitter and recombined at theoutput using an output 90° hybrid coupler 18 configured as a signalcombiner. The first PA 12 and the second PA 14 may be considered asbeing half-power amplifiers. The first PA 12 has a first PA input 20 anda first PA output 22. The second PA 14 has a second PA input 24 and asecond PA output 26. The input 90° hybrid coupler 16 has a first portinput 28 connected to a transmit terminal 30, which is labeled TXIN, anda second port input 32 connected to a fixed voltage node 34 through anisolation impedance 36, which is labeled RISO1. The fixed voltage node34 may be ground. The input 90° hybrid coupler 16 also has a third portoutput 38 connected to the first amplifier input 20 and a fourth portoutput 40 connected to the second amplifier input 24. The output 90°hybrid coupler 18 has a first port output 42 connected to a transmit outterminal 44, which is labeled TXOUT. The output 90° hybrid coupler 18also has a second port output 46 connected to the fixed voltage node 34through a second isolation impedance 48, which is labeled RISO2. A thirdport input 50 is connected to the second PA output 26 and a fourth portinput 52 connected to the first PA output 22. Impedance values for eachof the first isolation impedance 36 and the second isolation impedance48 is substantially equal to 50Ω, which results in excellent input andoutput return loss.

FIG. 2 is a diagram of a radio frequency front-end 54 that is structuredin accordance with the present disclosure. In contrast to therelated-art power amplifier 10, the radio frequency front-end 54 doesnot terminate the second port output 46. Instead, the second port output46 is configured to route a receive signal RXIN to a low-noise amplifier(LNA) 56. The LNA 56 has an LNA input 58 connected to the second portoutput 46 and an LNA output 60 connected to a receive output terminal62. The transmit out terminal 44 (FIG. 1 ) is replaced with a commonterminal 64 (i.e., RXOUT) that is configured to route the receive signalRXIN incident to the LNA 56 and output an amplified version of atransmit signal incident at the transmit terminal 30. Highly reflectiveimpedances |Γ| are presented at the third port input 50 and the fourthport 52, respectively. This causes an incident receive signal RXIN atthe common terminal to travel through the output 90° hybrid coupler 18,reflect off the highly reflective impedances |Γ|, and re-combine at thesecond port output 46, which is typically terminated in 50 ohms but isnow connected to the LNA input 58. Theoretically, a switching functionmay be implemented quite well without any switch devices whatsoever,assuming the output matching network loss of the first power amplifier12 and the second power amplifier 14 is low and output stage devices(not shown) comprising the first power amplifier 12 and the second poweramplifier 14 are configured to present the highly reflective impedancein when the first power amplifier 12 and the second power amplifier 14are turned off. For example, the output stage devices that are typicallyfield-effect transistors (FETs) are actually in the on-state whileperforming the receive function with drain voltages at 0 V and gatevoltages at 0 V.

Losses of output matching networks (not shown) of the first poweramplifier 12 and the second power amplifier 14 may result in too muchreceive loss because twice the matching network loss is added to thereceive loss along with twice the loss of the output 90° hybrid coupler18. FIG. 3 is a diagram of an embodiment of the radio frequencyfront-end 54 with a first shunt switch 66, a second shunt switch 68, anda third shunt switch 70 that are employed to increase receive-transmitisolation and reduce receive loss. Each of the shunt switches 66, 68,and 70 includes a first field-effect transistor (FET) labeled 1 to and asecond FET labeled 0. A logic level of one will turn on the first FET 1and turn off the second FET 0 and inversely a logic level of zero willturn off the first FET 1 and turn on the second FET 0. Each of the shuntswitches 66, 68, and 70 has a pole 72. The first FET 1 of each of theshunt switches 66, 68, and 70 is connected between the pole 72 and thefixed voltage node 34, so that the pole 72 is shorted directly to thefixed voltage node 34 when the first FET 1 is turned on. The second FET0 is connected between the fixed voltage node 34 and ground through ashunt capacitor 74, so that the pole 72 provides a capacitive impedancepath to the fixed voltage node 34 when the second FET 0 is on. Inexemplary embodiments, the fixed voltage node 34 is ground.

In the exemplary embodiment of FIG. 3 , the pole 72 of the first shuntswitch 68 is connected to a node between the first PA output 22 and thefourth input 52. The pole 72 of the second shunt switch 68 is connectedto a node between the second PA output 26 and the third port input 50.The pole 72 of the third shunt switch 70 is connected to a node betweenthe second port output 46 and the LNA input 58.

A controller 76 is configured to control switching of the first shuntswitch 66 and the second switch 68 by way of a first control signalCTRL1. The controller 76 turns the first FET 1 on and the second FET 0off for each of the first shunt switch 66 and the second shunt switch 68during a transmission operation by way of the control signal CTRL1.Inversely, the controller 76 turns off the first FET 1 and turns on thesecond FET 0 of the first shunt switch 66 and second shunt switch by wayof the control signal CTRL1 while receiving an RF signal through thecommon terminal 64. In the case that the first FET 1 and the second FET0 are perfect shorts, no loss is incurred on receive due to the outputmatching networks of the first power amplifier PA1 and the second poweramplifier PA2. Then the receive loss is limited to twice the loss of theoutput 90° hybrid coupler 18, which functions as a combiner. A fullanalysis of the losses due to the terminations and the excess loss ofthe output 90° hybrid coupler 18 are described subsequently.

The TX-RX isolation should be very good due to the inherent signalrouting of the balanced amplifier comprised of the first power amplifier12 and the second power amplifier 14, but inevitably some leakage to thesecond port output 46 connected to the LNA input 58 occurs in a transmitoperation. The isolation is improved further with the third shunt switch70 at the LNA input 58 that is turned on in transmit operation. Thisimproves the isolation at the expense of bandwidth and receive loss. Thecontroller 76 turns the first FET 1 on and the second FET 0 of the thirdshunt switch 70 off during transmission by way of a second controlsignal CTRL2. Inversely, the controller 76 turns off the first FET 1 andturns on the second FET 0 of the third shunt switch 70 by way of thecontrol signal CTRL2 while receiving an RF signal through the commonterminal 64.

In an implementation of the radio frequency front-end 54 in a front-endmodule (FEM), the transmit loss relative to a typical FEM issignificantly reduced due to the integration of the typical transmitswitching function within the normal balanced power amplifierarchitecture. This is a relatively large advantage of the embodiments ofthe present disclosure over a typical FEM.

In an incident signal at common port derivation, assume that the portsof the output 90° hybrid coupler 18 when operating as a power splitterare defined as follows:

P1: Input Port

P2: Isolated Port

P3: Through Port

P4: Coupled Port

The S-parameters of a 3 dB 90° hybrid coupler with excess loss definedas L are expressed as

$S = {\frac{- L}{\sqrt{2}}\begin{bmatrix}0 & 0 & j & 1 \\0 & 0 & 1 & j \\j & 1 & 0 & 0 \\1 & j & 0 & 0\end{bmatrix}}$

The linear factor L is defined such that it is actually a gain; forexample, it is a number between 0 and 1. Relating L to a loss that isdefined as a positive number of decibels, for example, a +3 dB loss,

$L = {10\frac{- L_{dB}}{20}}$

Assume a signal is incident at the input port (port 1), the isolatedport (port 2) is terminated in a matched load, the through port (port 3)is terminated in load Γ_(L), and the coupled port (port 4) is terminatedin load Γ_(L). The resultant incident (a_(i)) and reflected (b_(i))waves are expressed asb=Sa

In expanded form,

$\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix} = {{\frac{- L}{\sqrt{2}}\begin{bmatrix}0 & 0 & j & 1 \\0 & 0 & 1 & j \\j & 1 & 0 & 0 \\1 & j & 0 & 0\end{bmatrix}}\begin{bmatrix}a_{1} \\a_{2} \\a_{3} \\a_{4}\end{bmatrix}}$

Substituting the initial assumptions a₂=0, a₃=b₃Γ_(L), and a4=b₄Γ_(L),

$\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix} = {{\frac{- L}{\sqrt{2}}\begin{bmatrix}0 & 0 & j & 1 \\0 & 0 & 1 & j \\j & 1 & 0 & 0 \\1 & j & 0 & 0\end{bmatrix}}\begin{bmatrix}a_{1} \\0 \\{b_{3}\Gamma_{L}} \\{b_{4}\Gamma_{L}}\end{bmatrix}}$

Expanding,

$\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix} = {\frac{- L}{\sqrt{2}}\begin{bmatrix}{{{jb}_{3}\Gamma_{L}} + {b_{4}\Gamma_{L}}} \\{{b_{3}\Gamma_{L}} + {{jb}_{4}\Gamma_{L}}} \\{ja}_{1} \\a_{1}\end{bmatrix}}$

Based on foregoing equations,b ₃ =jb ₄

Substituting,

$\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix} = {{\frac{- L}{\sqrt{2}}\begin{bmatrix}{{{- b_{4}}\Gamma_{L}} + {b_{4}\Gamma_{L}}} \\{{b_{3}\Gamma_{L}} + {b_{3}\Gamma_{L}}} \\{ja}_{1} \\a_{1}\end{bmatrix}} = {\frac{- L}{\sqrt{2}}\begin{bmatrix}0 \\{2b_{3}\Gamma_{L}} \\{ja}_{1} \\a_{1}\end{bmatrix}}}$

Simplifying further,

$\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix} = {a_{1}\begin{bmatrix}0 \\{{jL}^{2}\Gamma_{L}} \\{{- j}\frac{L}{\sqrt{2}}} \\{- \frac{L}{\sqrt{2}}}\end{bmatrix}}$

Investigating the power at the isolated port,|b ₂|² =L ⁴|Γ_(L)|² |a ₁|²

The ratio of the incident power at the input port to the power leavingthe isolated port is then

$\frac{{b_{2}}^{2}}{{a_{1}}^{2}} = {L^{4}{\Gamma_{L}}^{2}}$

Investigating this equation, it is clear that the amount of power at theisolated port is related to the excess loss of the hybrid and to thereflection coefficient at the coupled and through ports. As an example,assume the output 90° hybrid coupler is implemented as a Lange coupleron a 100 μm silicon carbide (SiC) substrate. A typical loss for thisexemplary Lange coupler embodiment of the output 90° hybrid coupler isless than approximately 0.25 dB. Based on the foregoing equations, it isclear that the minimum amount of loss between the input and isolatedport is 0.5 dB due to the fourth power rather than second power on theloss exponent. That loss is only realized in the case in which purelyreflective terminations are presented at the through and coupled ports.The additional loss versus the magnitude of the reflection coefficientis plotted in FIG. 4 .

Clearly, the termination needs to be relatively good not to haveunacceptable loss. A plot more focused on the higher reflectioncoefficient terminations is shown in FIG. 5 .

This curve defines the usable region for the embodiments according tothe present disclosure. To keep the total loss less than 1 dB, thereflection coefficient magnitude needs to be greater than approximately0.94. In terms of implementing this reflection coefficient, likely ashunt switch in the on-state would be used, so it is useful to translatethe reflection coefficient to a resistance. This resistance versus theloss is shown in FIG. 6 , along with a focused version on the usefulregion shown in FIG. 7 .

For no more than 0.5 dB additional loss due to the termination, theshunt resistance needs to be less than approximately 1.5 Ohms. In agallium nitride (GaN) based implementation of the embodiment of FIG. 3 ,the on-resistance of the first FET1 and the second FET 0 is ˜1.3 Ohm×mm,so an 0.87 mm shunt device is sufficient to provide this shuntimpedance. To demonstrate this, the magnitude of the reflectioncoefficient for each of the first FET 1 and the second FET 0 in GaNbiased to the on-state is shown at both 30 GHz and 20 GHz in FIGS. 8 and9 , respectively. It is close to the numbers previously quoted fornecessary switch size, but the peripheries are slightly larger due tothe parasitic inductances of the first FET 1 and the second FET 0. At 30GHz, 984 μm of periphery is required to achieve a 0.94 reflectioncoefficient magnitude rather than the quick estimate of 870 μm. However,the initial estimate was within 13%. In some embodiments the first FET 1and the second FET 0 comprising each of the first shunt switch 66, thesecond shunt switch 68, and the third shunt switch 70 have a peripheryof substantially no greater than 1000 μm when operated with radiofrequency signals of substantially 30 GHz. The estimate is closer thelower the frequency is, due to the lower impact of the parasiticinductances.

This is well within the realm of possibility, even pushing into ka-band.Certainly it is feasible at frequencies lower than Ka band frequencies.It is necessary to absorb the capacitance of the shunt capacitor 74 inthe off-state into the power amplifier matching to ensure properoperation. At some point, the shunt capacitance becomes too large toabsorb and is bandwidth limiting.

An additional advantage of the architecture according to the presentdisclosure is the integration of the switching function with thebalanced amplifier, therefore reducing monolithic die or module size.The loss on transmit is reduced to a negligible amount while somepenalty is taken on receive due to the loss of the output 90° hybridcoupler 18. However, it is typically easier to increase the output powerof a front-end module than it is to reduce the noise figure, so it maybe a useful system trade-off to make.

It is contemplated that any of the foregoing aspects, and/or variousseparate aspects and features as described herein, may be combined foradditional advantage. Any of the various embodiments as disclosed hereinmay be combined with one or more other disclosed embodiments unlessindicated to the contrary herein.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A radio frequency front-end comprising: a firstpower amplifier (PA) having a first PA input and a first PA output; asecond PA having a second PA input and a second PA output; a low-noiseamplifier (LNA) having an LNA output connected to a receive outputterminal and an LNA input; an input 90° hybrid coupler having a firstport input coupled to a transmit terminal, a second port input connectedto a fixed voltage node through an isolation impedance, a third portoutput connected to the first amplifier input, and a fourth port outputconnected to the second amplifier input; an output 90° hybrid couplerhaving a first port output connected to a common terminal configured tooutput transmit signals and input receive signals, a second port outputconnected to the LNA input, a third port input connected to the secondPA output, and a fourth port input connected to the first PA output; anda first shunt switch coupled between the fourth port input and the fixedvoltage node, wherein the first shunt switch is configured to provide acapacitive impedance path to the fixed voltage node from the fourth portinput when the first power amplifier and the second power amplifier areamplifying a transmit radio frequency signal that arrives at thetransmit terminal.
 2. The radio frequency front-end of claim 1 whereinthe first power amplifier and the second power amplifier are configuredto present a reflective impedance |Γ| substantially equal to 1.0 whenthe first power amplifier and the second power amplifier are turned off.3. The radio frequency front-end of claim 1 wherein the first shuntswitch is further configured to short the fourth port input to the fixedvoltage node when the LNA is amplifying a receive radio frequency signalthat arrives at the common terminal.
 4. The radio frequency front-end ofclaim 1 further comprising a second shunt switch coupled between thethird port input and the fixed voltage node, wherein the second shuntswitch is configured to short the third port input to the fixed voltagenode when the LNA is amplifying the receive radio frequency signal thatarrives at the common terminal.
 5. The radio frequency front-end ofclaim 4 wherein the second shunt switch is further configured to providea capacitive impedance path to the fixed voltage node from the fourthport input when the first power amplifier and the second power amplifierare amplifying a transmit radio frequency signal that arrives at thetransmit terminal.
 6. The radio frequency front-end of claim 5 whereinthe capacitive impedance path presents a substantial open to thetransmit radio frequency signal.
 7. The radio frequency front-end ofclaim 4 further comprising a third shunt switch coupled between thesecond port output and the fixed voltage node, wherein the third shuntswitch is configured to short the second port output to the fixedvoltage node when the first power amplifier and the second poweramplifier are amplifying a transmit radio frequency signal that arrivesat the transmit terminal.
 8. The radio frequency front-end of claim 7wherein the third shunt switch is further configured to provide acapacitive impedance path to the fixed voltage node from the second portoutput when the LNA is amplifying the receive radio frequency signalthat arrives at the common terminal.
 9. The radio frequency front-end ofclaim 8 wherein the capacitive impedance path presents a substantialopen to the receive radio frequency signal that arrives at the commonterminal.
 10. The radio frequency front-end of claim 7 wherein the firstshunt switch, the second shunt switch, and the third shunt switch aregallium nitride (GaN) based.
 11. The radio frequency front-end of claim7 further comprising a controller configured to selectively controlswitching of the first shunt switch, the second shunt switch and thethird shunt switch.
 12. The radio frequency front-end of claim 7 whereineach of the first shunt switch, the second shunt switch, and the thirdshunt switch comprises a field-effect transistor configured as a switch.13. The radio frequency front-end of claim 12 wherein the field-effecttransistors comprising each of the first shunt switch, the second shuntswitch, and the third shunt switch have a periphery of substantially nogreater than 1000 μm when operated with radio frequency signals ofsubstantially 30 GHz.
 14. The radio frequency front-end of claim 7wherein a total loss of less than 1 dB is maintained during operationfor a reflection coefficient magnitude greater than approximately 0.94.15. The radio frequency front-end of claim 1 wherein the fixed voltagenode is ground.
 16. The radio frequency front-end of claim 1 wherein theoutput 90° hybrid coupler is implemented as a Lange coupler.
 17. Theradio frequency front-end of claim 1 wherein the Lange coupler isfabricated from a 100 μm silicon carbide substrate.
 18. The radiofrequency front-end of claim 17 wherein loss for the Lange coupler isless than approximately 0.25 dB.
 19. The radio frequency front-end ofclaim 1 wherein the isolation impedance provides substantially 50 Ω ofresistance.